Which of these numbers is prime? ${21,\ 29,\ 55,\ 56,\ 87}$
A prime number has exactly two factors: 1 and itself. The factors of 21 are 1, 3, 7, and 21. The factors of 29 are 1 and 29. The factors of 55 are 1, 5, 11, and 55. The factors of 56 are 1, 2, 4, 7, 8, 14, 28, and 56. The factors of 87 are 1, 3, 29, and 87. Thus, 29 is a prime number.